In recent years, there has been a rapid advance toward finer pattern rules for high integration and high speed performance of LSI devices. The application of shorter-wavelength exposure light sources is seen as one factor behind the advance to the finer pattern rules. For example, the wavelength reduction from mercury-lamp i line (365 nm) to KrF excimer laser radiation (248 nm) enables mass production of 64-Mbit DRAM (Dynamic Random Access Memory) (with a processing size of 0.25 μm or smaller). Further, the application of lithography process using ArF excimer laser radiation (193 nm) has been thoroughly studied for production of DRAM with an integration of 256M and of 1G or higher. In particular, the combination of ArF lithography process with a high NA lens (NA≧0.9) is being studied for production of 65-nm node devices. For production of next-generation 45-nm node devices, ArF immersion lithography process is being developed and put into use. Furthermore, double-exposure/double-patterning process based on ArF lithography and extreme ultraviolet (EUV) lithography process appear promising for 45-nm or smaller design rules.
As resist materials suitable for exposure to such short-wavelength radiation, attention is given to “chemically amplified resist compositions”. The chemically amplified resist composition is a pattern forming material that contains an acid generator (hereinafter referred to as a “photoacid generator”) capable of generating an acid by irradiation with high energy radiation (hereinafter referred to as “exposure”) and forms a resist pattern according to a photomask shape by causing a change in the developer solubility of exposed portions of the resist film through a reaction using the acid generated by exposure as a catalyst and thereby dissolving the exposed portions of the resist film.
It is required that the photoacid generator of the chemically amplified resist material has the characteristics that: the photoacid generator shows a high transparency to high energy radiation and a high acid generation quantum yield; and the generated acid shows a sufficiently high acidity, a sufficiently high boiling point and an adequate diffusion distance (referred to as “diffusion length”) in the resist film.
In the case of an ionic photoacid generator, the structure of the anionic moiety of the photoacid generator is an important factor for the acidity, boiling point and diffusion length of the generated acid. In the case of an ordinary, sulfonyl- or sulfonate-containing nonionic photoacid generator, the structure of the sulfonyl moiety of the photoacid generator is an important factor for the acidity, boiling point and diffusion length of the generated acid. For example, a photoacid generator having a trifluoromethanesulfonyl structure is capable of generating an acid of sufficiently high acidity so that the photoresist has sufficiently high resolution, but is disadvantageous in that the photoresist has high photomask dependence due to the low boiling point and long diffusion length of the generated acid. A photoacid generator having a sulfonyl structure with a large organic group bonded thereto, such as 10-camphorsulfonyl structure, is capable of generating an acid of sufficiently high boiling point and adequately short diffusion length so that the photoresist has low photomask dependence, but is disadvantageous in that the photoresist does not has sufficient resolution due to the insufficient acidity of the generated acid.
For these reasons, the chemically amplified resist composition for exposure to ArF eximer laser radiation generally uses a photoacid generator that generates a perfluoroalkanesulfonic acid of high acidity. However, the stability (non-degradability) of the perfluorooctanesulfonic acid and its derivatives due to C—F bonds as well as the biological concentration and accumulation of the perfluorooctanesulfonic acid and its derivatives due to hydrophobic and lipophilic natures have become problems. The above-mentioned problems are being raised against perfluoroalkanesulfonic acids of 5 or more carbon atoms. The U.S. Environmental Protection Agency has thus proposed a rule to regulate the use of these compounds.
Under the above circumstances, alkoxycarbonylfluoroalkanesulfonic acid onium salts such triphenylsulfonium methoxycarbonyldifluoromethanesulfonate (Patent Document 1), (4-methylphenyl)diphenylsulfonyl t-butoxycarbonyldifluoromethanesulfonate (Patent Document 2) and triphenylsulfonium (adamantane-1-ylmethyl)oxycarbonyldifluoromethanesulfonate (Patent Document 3) have been developed as acid generators that generate a partially- or fully-fluorinated, lower-carbon-number alkanesulfonic acid of sufficient acidity, adequate boiling point diffusion length and less environmental load.
In response to the demand for higher integration, there arises a need for not only the photoacid generator but also the photosensitive resin composition as the resist composition to address the requirement for higher resolution. The requirements for broader depth of focus tolerance (DOF) and smaller pattern line edge roughness (LER) are also increasing as finer patterning techniques progress. It is thus an urgent task to develop a resist composition that satisfies the requirements of high resolution, broad DOF, small LER, high sensitivity, good substrate adhesion and high etching resistance with the progress of the finer patterning techniques in the semiconductor industry.
In view of the foregoing, there have been reported a resin containing acroyloxyphenyldiphenylsulfonate as a copolymerization component for sensitivity improvement (Patent Document 4) and a base resin in which the above monomer is incorporated as a copolymerization component for LER improvement in polyhydroxystyrene (Patent Document 5). Each of these resins cannot however satisfy the above requirements as the cationic moiety is bonded to the base resin so that the sulfonic acid generated by irradiation with high energy radiation is the same as those generated from conventional photoacid generators. There has also been reported a sulfonate having an anionic moiety such as polystyrenesulfonic acid incorporated in its polymer chain for sensitivity and LER improvements (Patent Document 6). This sulfonate generates an arenesulfonic acid or alkylsulfonic acid derivative that is low in acidity and thus not enough to cleave an acid labile group, notably an acid labile group of a chemically amplified photoresist for use with ArF laser radiation.
Further, liquid immersion exposure is known in which each of a photoresist film applied to and formed on a resist wafer and a lens of a projection exposure system comes into contact with a liquid immersion medium such as water. In this exposure process, the pattern resolution of the photoresist film may be lowered by immersion of the liquid immersion medium into the photoresist film. There is also a problem that the surface of the lens may become dirty by elution of the photoresist component into the liquid immersion medium.
Fine processing using high energy radiation, such as extreme ultraviolet (EUV) radiation or charged particle radiation e.g. electron beam radiation, appears promising as the exposure process later than ArF lithography. In this fine processing process, exposure has to be carried out under vacuum (under reduced pressure) and thus causes volatilization of the sulfonic acid generated from the photoacid generator so that the resist pattern may not be formed with a good pattern shape and that the exposure system may be damaged by the volatilized sulfonic acid.
In order to solve the above problems, there have been developed sulfonates each having an anionic moiety incorporated in its polymer side and capable of generating a partially- or fully-fluorinated, lower-carbon-number alkanesulfonic acid (see Patent Documents 7 to 10). However, these sulfonates are produced from expensive raw materials by complicated production procedures and thus still have some problems left for industrial use.
Prior Art Documents
Patent Documents
Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-117959
Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-214774
Patent Document 3: Japanese Laid-Open Patent Publication No. 2004-004561
Patent Document 4: Japanese Laid-Open Patent Publication No. 4-230645
Patent Document 5: Japanese Laid-Open Patent Publication No. 2005-084365
Patent Document 6: Japanese Patent No. 3613491
Patent Document 7: International Publication No. WO 2006/121090 (PCT/JP/2006/309446).
Patent Document 8: Japanese Laid-Open Patent Publication No. 2006-178317
Patent Document 9: Japanese Laid-Open Patent Publication No. 2007-197718
Patent Document 10: Japanese Laid-Open Patent Publication No. 133448